Examination of the rotor head and its components indicates that the main rotor separated from the aircraft because the mast nut was not in place. The possibility that another mast nut was installed before the take-off and came loose in flight was rejected because all the mast nuts in inventory were found in the S.C.G. Hlicoptre inc. hangar. Consequently, the aircraft took off with no mast nut. A description of the work to be done should have been recorded on one of the documents, as required by regulation to advise maintenance personnel. Maintenance personnel could have referred to the documents and could have prevented the aircraft from taking off. However, the three persons who could have performed the work (the apprentice AME, the AME, and the Hliplan inc. owner) were aware that the mast nut had been removed and was to be painted on the evening of the occurrence. It is unlikely the AME thought that the mast nut had been installed by the apprentice AME or by his partner because the AME had received no notification or indication that the work had been completed. Consequently, it is reasonable to believe that the three persons who took part in the removal of the mast nut, and who were present when the aircraft took off, did not remember that the mast nut was in the hangar. It is unlikely that the helicopter would have taken off without the mast nut if a document had indicated the work that remained to be done and if the AME had consulted that document before the flight. There was no indication that the pilot or the AME consulted the aircraft documentation before the flight. It would have been unusual for the pilot to consult the maintenance documentation. The occurrence flight expedited the work to be done on the helicopter because it was not anticipated by the maintenance personnel. It had been decided that the droop restrainers and the mast nut would be painted that same day. It seems that, after a schedule change, the work methods of the maintenance personnel did not enable the AME to be aware of the airworthiness status of the aircraft at all time. It is likely that the pilot was not aware that the mast nut had been removed. Given that no visible warning device was placed in the cockpit or on the aircraft, there was nothing to tell the pilot that the aircraft was out of service. A visual aid such as a warning flag or sign, while not required by regulation, would have alerted the flight crew to the danger. Also, the missing mast nut undoubtedly would have been noticed by the pilot if a pre-flight check had been done as specified in the aircraft flight manual. Consequently, it is reasonable to conclude that the pilot did not climb atop the aircraft and did not examine the rotor head. Even if the restrainers were visible from the ground, noticing that something is missing is probably more difficult than noticing that something is present. The AME responsible for the maintenance of C-GFSE was on board the aircraft when it took off and had worked with the pilot during the hours preceding the flight; this certainly gave the pilot a false sense of security.Analysis Examination of the rotor head and its components indicates that the main rotor separated from the aircraft because the mast nut was not in place. The possibility that another mast nut was installed before the take-off and came loose in flight was rejected because all the mast nuts in inventory were found in the S.C.G. Hlicoptre inc. hangar. Consequently, the aircraft took off with no mast nut. A description of the work to be done should have been recorded on one of the documents, as required by regulation to advise maintenance personnel. Maintenance personnel could have referred to the documents and could have prevented the aircraft from taking off. However, the three persons who could have performed the work (the apprentice AME, the AME, and the Hliplan inc. owner) were aware that the mast nut had been removed and was to be painted on the evening of the occurrence. It is unlikely the AME thought that the mast nut had been installed by the apprentice AME or by his partner because the AME had received no notification or indication that the work had been completed. Consequently, it is reasonable to believe that the three persons who took part in the removal of the mast nut, and who were present when the aircraft took off, did not remember that the mast nut was in the hangar. It is unlikely that the helicopter would have taken off without the mast nut if a document had indicated the work that remained to be done and if the AME had consulted that document before the flight. There was no indication that the pilot or the AME consulted the aircraft documentation before the flight. It would have been unusual for the pilot to consult the maintenance documentation. The occurrence flight expedited the work to be done on the helicopter because it was not anticipated by the maintenance personnel. It had been decided that the droop restrainers and the mast nut would be painted that same day. It seems that, after a schedule change, the work methods of the maintenance personnel did not enable the AME to be aware of the airworthiness status of the aircraft at all time. It is likely that the pilot was not aware that the mast nut had been removed. Given that no visible warning device was placed in the cockpit or on the aircraft, there was nothing to tell the pilot that the aircraft was out of service. A visual aid such as a warning flag or sign, while not required by regulation, would have alerted the flight crew to the danger. Also, the missing mast nut undoubtedly would have been noticed by the pilot if a pre-flight check had been done as specified in the aircraft flight manual. Consequently, it is reasonable to conclude that the pilot did not climb atop the aircraft and did not examine the rotor head. Even if the restrainers were visible from the ground, noticing that something is missing is probably more difficult than noticing that something is present. The AME responsible for the maintenance of C-GFSE was on board the aircraft when it took off and had worked with the pilot during the hours preceding the flight; this certainly gave the pilot a false sense of security. The main-rotor head separated in flight because the mast nut was not in place. The helicopter took off without a mast nut. The pilot did not check the rotor head before the flight. Maintenance documentation did not indicate that the mast nut had been removed. No visible device was placed in the cockpit or on the aircraft to indicate that the helicopter was out of service. The three persons who participated in the removal of the mast nut were present when the aircraft took off. None of them remembered that the mast nut was not in place.Findings as to Causes and Contributing Factors The main-rotor head separated in flight because the mast nut was not in place. The helicopter took off without a mast nut. The pilot did not check the rotor head before the flight. Maintenance documentation did not indicate that the mast nut had been removed. No visible device was placed in the cockpit or on the aircraft to indicate that the helicopter was out of service. The three persons who participated in the removal of the mast nut were present when the aircraft took off. None of them remembered that the mast nut was not in place.